‘GUN-300’ is a new and distinct ‘McIntosh’ apple tree. It originated as a full tree mutation discovered near Crown Point, N.Y. in a cultivated area planted with ‘Rogers Red McIntosh’ (unpatented) trees, and was found to be distinguishable from ‘Rogers Red McIntosh’ and other known varieties. The new cultivar has been asexually propagated by budding and grafting near Louisana, Mo. and Hatford, Mich., and has been found to remain true to type through successive asexual propagations.